feat: Only build the nixosSystem for the install phase

Gate `nixosSystem` build behind the `installPhase` flag.
This ensures that the `nixosSystem` is only built when really needed by
`nixos-anywhere`.

This is especially useful if `nixpkgs.hostPlatform` is not set yet and
building the derivation would error out.
The build can be split in to phases with `nixos-facter` being run in the
kexec phase.

Potential Drawbacks:

Since the `nixosSystem` is not built early in the invocation we can't
fail fast on errors anymore.
But this is only the case if the phases are already controlled from the
"outside" eg. if `nixos-anywhere` is called without the install phase
active.

Author: a-kenji

src/nixos-anywhere.sh

@@ -726,7 +726,9 @@ main() {
       if [[ ${phases[disko]} == 1 ]]; then
         diskoScript=$(nixBuild "${flake}#${flakeAttr}.system.build.${diskoMode}Script")
       fi
-      nixosSystem=$(nixBuild "${flake}#${flakeAttr}.system.build.toplevel")
+      if [[ ${phases[install]} == 1 ]]; then
+        nixosSystem=$(nixBuild "${flake}#${flakeAttr}.system.build.toplevel")
+      fi
     fi
   elif [[ -n ${diskoScript} ]] && [[ -n ${nixosSystem} ]]; then
     if [[ ! -e ${diskoScript} ]] || [[ ! -e ${nixosSystem} ]]; then
@@ -799,7 +801,9 @@ main() {
     if [[ ${phases[disko]} == 1 ]]; then
       diskoScript=$(nixBuild "${flake}#${flakeAttr}.system.build.${diskoMode}Script")
     fi
-    nixosSystem=$(nixBuild "${flake}#${flakeAttr}.system.build.toplevel")
+    if [[ ${phases[install]} == 1 ]]; then
+      nixosSystem=$(nixBuild "${flake}#${flakeAttr}.system.build.toplevel")
+    fi
   fi
 
   # Installation will fail if non-root user is used for installer.